1. Field of the Invention
This invention relates to molecular-engineered porous silica materials having micropores in a narrow size range. In addition, this invention relates to a process for the production of molecular-engineered porous silica materials having micropores in a narrow size range produced through the selective introduction of hydrocarbon templates into the silica structure and their subsequent removal from the solid material structure.
2. Description of the Prior Art
Porous silica has a variety of uses including desiccating water from closed packages, providing the solid phase for chromatographic separations, and acting as a support for catalysts used in numerous industrial processes. Silica is, by definition, an amorphous, glass-like material whose characteristics, including porosity, are primarily dependent upon the reaction and/or processing conditions used in its preparation. Porosity in silica gels is particularly important for the applications described above in which large surface areas are important for the processes being carried out. Large silica surface areas (meter-squared per gram of material) can translate into greater quantities of water or contaminant being adsorbed by silica desiccants, higher resolution separations of chemical mixtures through chromatography, and more efficient supported catalysts. However, the pore sizes in silica gels prepared by conventional techniques may range in size from macropores (&gt;500 nm mean pore diameter) to micropores (&lt;2.0 nm). Such broad and ill-defined porosity in silica prevents its application as a molecular sieve. Amorphous silica with the capability to perform molecular sizing in a similar fashion to zeolitic molecular sieves would be of enormous practical use considering the ease with which silica can be prepared in large monolithic shapes or in thin films.
Whereas porosity in zeolites arises from the crystalline arrangement of the network material, porosity in silica materials arises from interconnecting voids dispersed throughout the material. While traditionally the size of these voids can be manipulated by varying the reaction and processing conditions, the objective of the hydrocarbon template approach described here is to introduce porosity into the material based on the length of the hydrocarbon template or spacer.
It is known to prepare microporous aryl-bridged polysilsequioxanes by sol-gel processing of monomers such as bis-1,4-(triethoxysilyl)benzene and bis-4,4'-(triethoxysilyl)biphenyl, and their bis(trichlorosilyl) analogs. The materials produced by hydrolysis and condensation of these monomers result in rigid-rod organic spacers interspaced at regular intervals in the silicate-like framework. The xerogels produced upon subsequent processing of the gels have extremely high surface areas with porosities confined to the micropore range. The preparation of such aryl-bridged polysilsesquioxanes is previously described by the inventors in "Aryl-Bridged Polysilsequioxanes-New Microporous Materials," Chemistry of Materials, vol. 1, p. 572,(1989) and by Shea et. al. in "Aryl-Bridged Polysilsesquioxanes-New Microporous Materials, Better Ceramics Through Chemistry," Mater. Res. Soc. Symp. Proc., vol. 180, p. 975,(1990).
The present invention overcomes deficiencies of prior art silicas by providing molecular engineered porous silica materials having narrowly defined micro-porosity, allowing their use as molecular sieves, large surface area silica desiccants, efficient supported catalysts, and for high resolution separation of chemical mixtures through chromatography. This is accomplished by the manipulation of the porosity of silica using one or more of a series of organic template groups covalently incorporated into the silicate matrix. The templates in the bridged polysilsesquioxanes are selectively removed from the resultant material, leaving engineered voids or pores. The size of these pores is dependent upon the length or size of the template or spacer. The size of the templates is measured in terms of Si-Si distances ranging from about 0.67 nm to about 1.08 nm. Changes introduced by the loss of these templates are in the micropore range of &gt;2 nm.
Although the previous work of the inventors as discussed above provided for microporous aryl-bridged polysilsesquioxanes, they have the disadvantage of having inconsistently sized micropores, resulting in ill-defined porosities. It has been found that the step of removal of the aryl templates from the material by means of oxidation such as through the use of oxygen plasma or ozone results in stable, well-defined porosity material capable of predictable performance for the uses described above.
It has further been found that alkyl-bridged polysilsequioxanes, although nonporous in the intermediate stage, become porous upon the carrying out of the inventive step of removal of the hydrocarbon templates. The removal of the alkyl templates is carried out in the same manner as the aryl templates described above, e.g., through oxidation using oxygen plasma or ozone. The use of alkyl-bridged polysilsesquioxanes using hexyl or nonyl alkyl groups may be advantageous over the aryl-bridged polysilsesquioxanes in that the precursors are more easily prepared. Further, it has been found to be advantageous to prepare non-porous material with the ability to treat it according to the invention at a later time or in the environment which it is to be used to render it porous. It has also been found that aryl-bridged polysilsesquioxanes can be made exhibiting limited or non-porosity until treatment according to the invention and, thus, exhibit the same advantage as alkyl-bridged polysilsesquioxanes.